Washing machines may include motors that can output low-speed and high-torque during a wash operation and high-speed and low-torque during a dehydrate operation. In order to meet these opposite requirements, motors are generally designed to possess properties that are in between the two extremes. In order to further accelerate the rotation speed during the dehydrate operation, techniques have been conceived to suppress induced voltage by executing a field weakening control in instances where the induced voltage of the motor exceeds the power supply voltage of the inverter circuit. However, because this approach introduces increase in motor current, degradation in motor efficiency is inevitable.
Patent publication 1, for instance, discloses a technique for varying the motor output properties by providing two ring-shaped permanent magnets at the rotor of a brushless DC motor and rotating one of the magnets to vary the field magnetism through change in the position of magnetic pole. The disclosed technique, however, requires a configuration to rotate either of the permanent magnets and thus, significantly confines the structure of the rotor.
Patent Publication 2, on the other hand, discloses a technique of configuring a permanent magnet motor by combining a high coericivity neodymium magnet and a low coercivity alnico magnet and executing a high-speed rotation which increases/decreases the magnetic flux of the alnico magnet to reduce the total linkage flux obtained by the sum of the two types of magnets.    Patent publication 1: JP H10-155262 A    Patent publication 2: JP 2006-280195 A